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I just finished up a unit of 6 Seraphim to add to my small Witchhunters army.
Right now I have
1 Inquistor squad with enough models to make different configurations
2 10 unit of IST
So what should I get next. I think my options are
10 Battle Sisters
If I get the battle sisters what should I arm the special weapons troops with. Also should I make them into celestians or leave them as plain battle sisters.
From the look of your list, you only have 3 faith points not including the litanie if you have purchase that.
I would suggest you to go for the 10 Battle Sisters, but it would not make much of a difference if you only have 10, you'd need at least 20 to make a difference. If you are going to get the 10 Sisters in the end, make them into a unit of Celestian for starter, arm them with either Meltagun/Flamer (hvy flamer if point permits) if you have the transportation option or 2x Storm Bolters if you are letting them walk, give the VSS an eviscerator and you'll have a 2 attacks chainfist that hits on 3's and wound on 2's.
Another good option is to take the Exorcist, but then again, I hate to do this to you but a single Exorcist will put you in alot of disappointments. Two is the minimal number, you either take 2 or you take none, that's the way I play.
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If it were me, I'd invest in some more Battle Sisters. These girls can end up being the mainstay of your army, and having a good pool of them means that you can seriously deal out the faith in the game. Its fun watching a unit of berserkers fall in close combat to a unit of Battle Sisters.
I'd also grab some more seraphim, they tend to perform better in larger groups.
WH armies seem to go into two different directions – either you go Sisters of Battle all the way, or you use IST and some IG or maybe SM to support your Inquisitor. From your list, it looks like you went the Inquisitor/IST/IG route. Maybe get Chimeras for your ISTs? If you want sisters, make them into Celestians as suggested above – after rapid firing they stand a chance of surviving the unavoidable countercharge…
If you can get the models, you could get a 10 strong sister squad in a rhino and then a small 5/6 man celestian squad in a immy or just on foot. These would give you some extra and valuable faith points.